Ruby regular expression using variable name

It works, but you need to use gsub! or assign the return to another variable

var = "Value"
str = "a test Value"
str.gsub!( /#{var}/, 'foo' )  # Or this: new_str = str.gsub( /#{var}/, 'foo' )
puts str

The code you think doesn't work, does:

var = "Value"
str = "a test Value"
p str.gsub( /#{var}/, 'foo' )   # => "a test foo"

Things get more interesting if var can contain regular expression meta-characters. If it does and you want those matacharacters to do what they usually do in a regular expression, then the same gsub will work:

var = "Value|a|test"
str = "a test Value"
str.gsub( /#{var}/, 'foo' ) # => "foo foo foo"

However, if your search string contains metacharacters and you do not want them interpreted as metacharacters, then use Regexp.escape like this:

var = "*This*"
str = "*This* is a string"
p str.gsub( /#{Regexp.escape(var)}/, 'foo' )
# => "foo is a string"

Or just give gsub a string instead of a regular expression. In MRI >= 1.8.7, gsub will treat a string replacement argument as a plain string, not a regular expression:

var = "*This*"
str = "*This* is a string"
p str.gsub(var, 'foo' ) # => "foo is a string"

(It used to be that a string replacement argument to gsub was automatically converted to a regular expression. I know it was that way in 1.6. I don't recall which version introduced the change).

As noted in other answers, you can use Regexp.new as an alternative to interpolation:

var = "*This*"
str = "*This* is a string"
p str.gsub(Regexp.new(Regexp.escape(var)), 'foo' )
# => "foo is a string"

Yes

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str.gsub Regexp.new(var), 'foo'